It is a general requirement to design a radio frequency power amplifier such that the radio frequency power amplifier is reasonably linear, is highly efficient and is substantially insensitive to load impedance variation and component variation or mismatching load impedances. Power efficiency, for example, expressed as the ratio of delivered RF signal power (Pout) Dc input power consumed. Power amplifiers are needed at both ends of cellular and other wireless communication: At the base station and at the mobile terminal. Base stations typically need to process signals of multiple users. Therefore the total RF signal they need to transmit—even when applying Crest factor reduction techniques—typically exhibits a peak to average power ratio (PAPR, or Crest factor) of 7-10 dB. This means that the average output power is 5 to 10 times lower than the peak instantaneous power. With modern communication standards, such as WiFi or LTE, even the mobile terminal suffers from high PAPR. Conventional power amplifiers like class-A, class-AB etc. are quite inefficient for high PAPR signals. For cellular base-stations, a more efficient architecture is the Doherty amplifier. However, this architecture is narrowband, and very sensitive to variations in load impedance. Another architecture is the class-S amplifier, a switch-mode amplifier driven with a band-pass delta-sigma modulated quasi-digital signal. In theory, this amplifier can reach 100% efficiency. In practice, due to parasitic capacitances and resistances of real transistors, it is not very efficient, and inter symbol interference distorts the output spectrum.
A cleaner spectrum which is less sensitive to inter-symbol interference is provided by the RF out-phasing amplifier. However, efficiency is only moderate for high PAPR signals, because switching frequency and switching loss do not reduce at reduced (backed-off) instantaneous output power.
It would be desirable to provide an RF power amplifier which can at the same time provide high power efficiency and a reasonably clean spectrum (low ACLR) in presence of high PAPR signals.
It would further be desirable to provide an RF power amplifier that is not very sensitive to variations in load impedance.